Discharge tube



Nov. 14, 1950` J, L, H', JONKER ETAL 2,530,167

DISCHARGE TUBE Filed March l, 1947 CATHODE JOHAN LODEWIJK HENDRIK JONKER8 EDUARDGERARDUS DORGELO AGENT Patented Nov. 14, 1950 DISCHARGE TUBEJohan Lodewijk Hendrik Jonker and Eduard Gerardus Dorgelo, Eindhoven,Netherlands, as-

, signers to Hartford National Bank and I rust Company, Hartford, Conn.,as trustee Application March 1, 1947, serial No. 731,802 In BelgiumDecember 14, 1944 Section l., Public Law 690, August 8,1946

` Patentexpires December 14, 1964 Thisinvention` relates to an electricdischarge tube the electrode system of which comprises at least onecathode vand one anode. 'The invention is more particularly concernedwith the improvement in the'thermal radiation properties and theselection of secondary emission properties of electrodes that are notcoated with primarily emitting substances. Further the invention relatesto a method of coating anl :electrode with a layer possessed of theproperties referred to.

It is an established fact that in discharge tubes of the aforesaid kindall the electrodes assume a higher temperature than they have when thetube .is put into use under normal conditions. This heating is due bothto the normal thermal radiation of the cathode and to the constantbombardment by electrons. Once an electrode has assumed a giventemperature it may in turn also heat the adjacent electrodes byradiation. As a matter of course, this heating is greatest in tubesoperating with heavy currents, such as transmission valves. V

The-effects of the heating of the various electrodes are of two kinds.Firstly', the mechanical strength of almost all of the usual materialsdecreases at an increased temperature. Secondly, the secondary emissionfactor increases with the temperature. The former fact is alwaysdetrimental while the latter may or may not be an adverse factordepending upon the purpose for which the device is designed.Accordingly, precautions have previously been suggested to obviate thesedifliculties.

These precautions can be divided into two groups, to wit: into thoseoperative to prevent the cause, that is to say excessive heating, andthose operative to remove the effects or else to check them.

The second group includes for example the selection of materialsrefractory to a higher extent and of materials having a low secondaryemission coeicient. The invention is however concerned with precautionspertaining to the rst group.

It is known that the rise in temperature can be retarded by the use or"artificial cooling, for example by :a flow of liquid, by the use ofcooling ns and by coating the surface of the pertinent electrodes with asubstance that improves the thermal radiation. It has previously beensuggested to constitute the coating by a thin layer of car-bon, forexample in the form of soot, or else a layer of a black oxide or anoxide in a black form, for example titanium oxide may be utilized. Asthe secondary emission generally s claims.y (01.250-174) decreases atlower temperatures, it will fall due to the improved thermal radiationof the 'elec' trodes and the consequent lower temperature thereof; mostofthe aforementioned good radi'- ating. substances, particularly carbon,however,

also have a lowl secondary emission factor so that the secondaryemission is likewise reduced materially. To sum` up-the matter shortlyit cany therefore be established that coating an elec- 1 trode with agood heat Aradiating substance generally results in a decrease of thesecondary emission. In some cases however, it is desirable that whilstincreasing the radiation of an electrode, so that by. means of theconsequent fall in temperature the mechanical strain on the elec-r trodematerial may be lessened, the secondary emission be maintained; or elsevnotfnullifiedr 'ene' tirely and inl somefcasesfmay even be desirable toincrease it. The invention' has forits'object to ensure this proposedvpurpose.

An electric discharge tube according to the invention comprisesaprimarily emitting cathode and one or'more' other electrodes, one ofwhich' at least is coated entirely or in part 'with-.al mixture of agood heat radiating material and' a substance the secondary emissioncoefficient of which equals or exceeds 1.

Apart from the fact that the temperature of the coated electrodes, inspite of a high vload of the tube, remains so low that there is no riskof undue alterations in the functions of the electrodes, for example dueto changes in the position relatively to the other electrodes, theinvention has the important advantage that the secondary emission can beimparted a given value by the choice of a suitable substance. This isparticularly important in connection with those electrodes which whilethe tube is in use must preferably not take up current, for examplecontrol grids. As is well-known, considerable damping is set up in atuned circuit connected to a control grid, ifrcurrent from the grid hasto be conducted away through the circuit. Now, it is possible for thesecondary emission of the electrodes concerned to be such that thecurrent from the electrodes is equal to zero or if desired is evennegative.

It is already known to cause the current passing to a grid to be equalto zero by the'use of secondarily emitting substances. It was howeverhitherto impossible to improve the thermal radiation of an electrode bya coating of the surface whilst at the same time imparting a given valueto the secondary emission.

Further it is known to coat an electrode of an electric discharge tubefor the purpose of improving the thermal radiation with a layer ofcarbon, the carbon layer and the electrode being separated by a layermadeup of silicates or oxides of beryllium, magnesium or aluminium. Thislayer prevents the detrimental absorption of the carbon in the electrodebut no question of a mixture of carbon and the silicates or oxidesarises. The secondary emission of suchfan electrode will accordingly bepractically entirely governed by the carbon layer, the oxide or silicatelayer not being on the surface.

It is only natural that the phenomena of grid current, secondaryemission, high mechanical strain, and so forth more readily occur withtubes operating with high voltages, for example, transmission valves, sothat the invention is particularly useful in connection with tubes ofthis kind.

In a tube according to the invention the good heat radiating substancemay be carbon, for example in the form of soot or of black oxides oroxides in a black form due to grain roughness, for example titaniumoxide, zirconium oxide, tungsten oxide, molybdenum oxide, this substancebeing mixed with thegood secondarily emitting substance, which may beconstituted, for example, by magnesia, because this is known to have agood secondary emission coeicient and to be secure against hightemperatures. Another substance that may be used to obtain goodsecondary emission is beryllium oxide. The black substance and the goodemitting substance are mixed together, for example applied from asuspension to the surface of the electrode, the suspension agent beingthen removedl for example by heating the electrode. The choice of theratio between the two substances permits of ensuring a given secondaryemission factor within certain limits.

In view of an accurate control of the heat radiating and secondaryemission properties part of the electrodes may be left uncoated or maybe coated merely with secondarily emitting material or merely withthermal radiating substance.

What we claim is:

1. An electric discharge tube comprising a primarily emitting cathodeand at least one other electrode coated with a mixture of a good heatradiating material and a substance the secondary emission coeicient ofwhich equals at least 1.

2. An electric discharge tube as claimed in claim l, wherein the goodheat radiating substance is constituted by carbon.

3. An electric discharge tube, as claimed in claim 1, wherein the goodheat radiating substance is constituted by a black metal oxide.

4. An electric discharge tube, as claimed in claim 1, wherein the goodheat radiating substance is constituted by a metal oxide in a blackform.

5. An electric discharge system as claimed in claim l, wherein the goodheat radiating substance is constituted by titanium oxide.

6. An electric discharge tube as claimed in claim 1, wherein thesubstance the secondary emission coefficient of which exceeds 1 isformed by oxides of the group consisting of magnesium and beryllium.

7. An electric discharge tube as claimed in claim 1 wherein thesubstance whose secondary emission coeicient exceeds 1 is magnesia.

8. An electric discharge tube comprising a primarily emitting cathodeand at least one other electrode coated with an intimate, uniformmixture of soot and magnesia.

JOHAN LODEWIJ K HENDRIK J ONKER. EDUARD GERARDUS DORGELO.

REFERENCES CITED The following references are of record in the tile ofthis patent:

UNITED STATES PATENTS Number Name Date 2,012,339 Edwards et al Aug. 27,1935 2,233,917 De Boer et al Mar. 4, 1941 FOREIGN PATENTS Number CountryDate 473,398 Great Britain Oct. 12, 1937 492,025 Great Britain Sept. 13,1938

